IMMUNITY AND SUSCEPTIBILITY. 83 



amount of toxin endurance of a resistent animal be repre- 

 sented by x, and any addition to this as y. Then xy 

 would certainly be fatal. If the least quantity of anti- 

 toxin that will protect the animal be expressed by 2, then 

 xy + z is harmless. It is evident, however, that z does 

 not necessarily have any influence upon x, but only need 

 neutralize y in order to save the animal, and therefore 

 it is obvious that the remaining x in such a mixture 

 could readily destroy another more susceptible animal 

 into which it might be injected. 



I am of the opinion that the effect of the antitoxin 

 really partakes of the nature of chemic neutralization 

 from the following experiment: let x represent the least 

 certainly fatal dose of diphtheria toxin for a guinea-pig, 

 and y the least quantity of antitoxin that will protect 

 against it; then 



x -f- y is harmless. That 



10 x + 10 y is also harmless is known to every one 

 accustomed to test antitoxins. I have con- 

 tinued this and have found that 



50 x -f 50 y 

 100 x + 100 y are also harmless. 



According to Buchner, the antitoxins differ from the 

 alexins in being new substances in the blood, in being 

 without germicidal or chemical neutralizing power against 

 the toxins, and in being stable compounds which can 

 resist heat to 75 C., can resist a reasonable amount of 

 exposure to light, and which are not altered by decompo- 

 sition of the substances containing them. 



The antitoxins are specific for one poison only. Ehrlich 

 found that antiricin was powerless against abrin, and vice 

 versa. Diphtheria antitoxin is of no avail against tetanus, 

 and vice versa. 



The immunity which the antitoxins produce is fuga- 

 cious, varying considerably according, to the particular 

 substance employed. As a rule, it is limited to a few 

 months at least in the case of such antitoxins as we can 

 produce experimentally. 



